A switching power supply turns on or turns off a switch to control a current flowing through a primary winding of a transformer, and rectifies and smoothes a voltage developed across a secondary winding of the transformer, to supply the resulting current output to a load. Since the transformer used in such a switching power supply serves to transfer energy, a structure and a characteristic of the transformer play important roles.
FIG. 1 is a view showing a structure of one example of a magnetic leakage transformer disclosed in Japanese Patent Application Laid-open No. 2000-340441. The magnetic leakage transformer 111 shown in FIG. 1 is comprised of an E-core 113, formed with a magnetic circuit, which is made of magnetic material, an I-core 115 that forms a main iron core together with the E-core 113, a primary winding 119 and a secondary winding 125 mounted to the E-core 113 at suitable locations, a cylindrical magnetic leakage iron core 123, made of magnetic material, which is located in a position through which magnetic fluxes leaked from the magnetic circuit pass, and a current detection winding 121 mounted onto the magnetic leakage iron core 123 for detecting leakage magnetic fluxes.
In the magnetic leakage transformer 111, since the magnetic leakage iron core 123 made of magnetic material is located in a position through which magnetic fluxes leaked from the magnetic circuit pass and the current detection winding 121 is mounted onto the magnetic leakage iron core 123 for detecting the leakage magnetic fluxes, the current can be detected without causing power loss and complexity in structure.